Mobile: debounce live-phase changes by 250ms

Status summary#

Done and merged via PR #2549: generic useDebouncedValue hook (250ms trailing debounce, first-value seeding), Feather icons + shared row geometry, all bot findings addressed.

What#

The live activity card's phase glyph (#2543) still shows sub-100ms flashes: the journal-derived certainty signals cover the KNOWN owed-turn gaps, but fast event ripples (settle + follow-up landing near-simultaneously, chunk boundaries) can still flip the derived phase for a frame or two. Misha's ask: debounce the displayed phase by 250ms — when the derived phase changes, queue the new value; if nothing else changes for 250ms, switch; if it changes again first, restart the wait (trailing debounce). A flap A→B→A within the window shows A throughout.

How (decided)#

• Presentation-layer only: the reducer stays a pure fold (no clocks). The debounce lives in a mobile hook.
• No useState/useEffect (house rule): useQuery keyed on the derived status's content — queryFn resolves the new value after 250ms, placeholderData: keepPreviousData shows the old value meanwhile, and staleTime: Infinity makes a return to a cached phase instant (flap suppression for free).
• Debounce the whole AgentUiLiveStatus (phase + statusText) as one unit — text flashes are as jarring as icon flashes.
• First value on card mount displays immediately (nothing to flash from).
• liveStatus === null (settled/absent) passes through undebounced — the card's live→settled switch is journal-final.

Checklist#

• useDebouncedLiveStatus(activityId, liveStatus) hook in apps/mobile/src/lib/ (tanstack-query debounce as above).
• chat.tsx: pass the debounced status to the live ActivityCard.
• Existing specs/mobile/live-status.spec.ts still green (its phase holds are all ≥4s, far above the debounce; the lag is absorbed by the waits).
• Sanity-run against local dev. (spec 27.4s, all phases assert through the lag)

Assumptions made (Misha was around but this is small)#

• 250ms constant, not configurable.
• The WorkingCard (pre-first-event ⧗) is not debounced — it appears from nothing and hands over to a card whose first phase renders immediately; both edges are single transitions with nothing to flap against.
• No new unit-test infra for the hook: mobile has no component-test lane, and the observable contract is covered by the spec still passing plus the hook being ~20 lines of query wiring. If a mobile dom-test lane appears later, a flap-suppression test belongs there.

Implementation log#

(append as you go)